1. Field of the Invention
The present invention generally relates to purging process lines that contain process gases used in semiconductor wafer fabrication, and more particularly to purging process gas having a relatively low vapor pressure from a process line by charging inert gas into the process line and evacuating the process line without liquefying the process gas.
2. Description of the Related Art
In some semiconductor chip fabrication processes (e.g., chemical vapor deposition, physical vapor deposition, etching), it is required that selected gas delivery process lines be maintained free of air. Such process lines are periodically purged to eliminate contaminants (e.g., atmosphere) from the line. Process lines usually must be purged when the flow of process gas through the line ceases, such as when an exhausted process gas source is replaced. Purging a process line typically includes repeatedly charging the line with inert gas and evacuating the line with a vacuum system.
The process gases used in many semiconductor chip fabrication processes frequently have relatively low vapor pressures. If inert purge gas is introduced into a process line at a pressure greater than the vapor pressure of the process gas, the process gas tends to liquefy in the line. The purging of liquefied process gas from a process line is often difficult or impossible. Liquid contained in the process line may also inhibit the functioning of mass flow controllers contained in the line. In addition, many process fluids are especially corrosive in a liquid state, and the presence of such liquids may require the replacement of one or more process lines. Attempts have been made to prevent the formation of liquids in process lines by externally heating the lines when purge gas is introduced into the lines. Such methods, however, tend to be expensive and generally do not prevent process gas from liquefying when the pressure of the purge gas significantly exceeds the vapor pressure of the process gas.
It is therefore desirable that an improved purge system and method be derived to prevent the formation of liquids when inert gas is introduced into process lines during purging.
In accordance with the present invention, an inert gas purge system and method is provided that largely eliminates or reduces the aforementioned disadvantages of prior methods. An embodiment of the invention relates to a purge system that includes a process conduit for directing a process gas from a process gas source to a process unit. The process conduit preferably communicates with a purge conduit that is connected to an inert gas source. Inert gas is preferably directed from the inert gas source through the purge conduit and into the process conduit. The purge conduit preferably contains a pressure restriction device for reducing the pressure of the inert gas below a predetermined pressure prior to the introduction of the inert gas into the process conduit. The purge system preferably further includes a vacuum system for evacuating the process conduit of inert gas and residual process gas. The vacuum system may include an ejector and a vacuum conduit communicating with the process conduit. The process conduit may be repeatedly charged with inert gas and evacuated until the concentration of process gas within the process conduit falls below a predetermined level.
The pressure reduction device preferably reduces the pressure of the inert gas below the vapor pressure of the process gas to ensure that the pressure within the process conduit remains below the vapor pressure of the process gas. The inert gas is preferably introduced into the process conduit at a predetermined pressure such that substantially no residual process gas liquefies within the process conduit. The process gas may include dichlorosilane, trichlorosilane, tungsten hexafluoride, boron trichloride, carbon tetrachloride, hydrogen fluoride, and/or octafluorocyclobutane. The inert purge gas is preferably nitrogen.
The pressure reduction device is preferably a set (i.e., non-adjustable) pressure regulator. The pressure reduction device may reduce the inert gas pressure from a pressure in excess of about 30 psig to a pressure of about 5 psig or less. The pressure reduction device preferably reduces the pressure of the inert gas to between about 1 psig and about 15 psig prior to its introduction into the process conduit.
The purge system is preferably included in a gas tray system and/or gas panel system used to deliver process gas to a process unit. The process unit may be a chemical deposition chamber, physical vapor deposition chamber, etching chamber, or other unit that uses process gas in a semiconductor wafer manufacturing process.
An advantage of the invention relates to charging an inert purge gas into a process conduit without liquefying residual process gas contained in the process conduit.